1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a copier, a facsimile apparatus or the like, for forming a recorded image by developing an electrostatic latent image formed on an image bearing member according to an electrophotographic method, an electrostatic recording method or the like, using a developer, and then transferring the developed image onto a recording material, a cartridge detachably mountable in the image forming apparatus, and a storage medium to be mounted in the cartridge.
2. Description of the Related Art
For example, in an image forming apparatus adopting an electrophotographic method, in general, a toner image is formed by causing a toner of a charged developer to be attracted onto an electrostatic latent image formed on the surface of an image bearing member. Then, the toner image is transferred onto a recording material conveyed so as to contact the image bearing member, directly or after first transferring the toner image from the image bearing member onto an intermediate transfer member, and thereafter performing heating/fixing processing to complete image formation on the recording material.
Usually, a so-called two-component developer obtained by mixing a magnetic powder called a “carrier” and a “toner” consisting of colored particles is used as the developer. Particularly, in color image forming apparatuses, since the carrier to which the toner adheres can be conveyed to the image bearing member by magnetically holding the carrier on a developer carrying member, without adding a magnetic material in the colored particles, the two-component developer is preferably used from the viewpoint of a hue and the like. In the two-component developer, the toner is charged by friction between the toner and the carrier, and only the toner is caused to be attracted onto an electrostatic latent image formed on the surface of the image bearing member. Accordingly, if image forming processing is executed, the amount of the toner in the developer decreases. That is, the density of the toner in the developer (the ratio of the toner in the entirety of the developer, or the ratio of the amount of the toner to the amount of the carrier) changes.
Accordingly, an image forming apparatus using a two-component developer includes means for detecting the density of the toner in the developer within the main body of a developing device (developing receptacle). When the amount of the toner remaining within the developer receptacle becomes less than a predetermined (constant) value, a toner replenishing operation from a toner replenishing device is executed.
When using a two-component developer, by using automatic toner-replenishment control means ATR (automatic toner replenisher) for performing appropriate toner replenishment in accordance with a variation in the toner density by timely detecting the toner density in the developer, the toner density must always be maintained within a constant tolerance with respect to a predetermined reference value.
The automatic toner-replenishment control means usually includes toner-density detection means for detecting the toner density in the developer, toner-replenishment control means for determining the amount of toner replenishment by processing output data from the toner-density detection means, and toner replenishing means for replenishing the toner based on the amount of toner replenishment determined by the toner-replenishment control means. Particularly, various types of toner-density detection means have been put into practical use.
For example, there are types of toner-density detection means that: utilize the fact that the optical reflectivity of the developer within the developing receptacle or the developer bearing member changes depending on the toner density; utilize a permeability sensor for converting permeability into an electric signal by utilizing the fact that the permeability of the developer changed depending of the toner density; indirectly estimate the toner density in the developer by detecting a change in the optical reflectivity of a predetermined patch image formed on a latent-image bearing member in predetermined conditions; and the like.
In image forming apparatuses of a type that form a digital latent image using a laser scanner or an LED (light-emitting diode) array, since the amount of toner consumption per page can be relatively correctly estimated from the accumulated value (the number of video counts) of the number of printed pixels in an image information signal per page, automatic toner-replenishment control means of a type of determining the amount of toner replenishment in accordance with the estimated amount of toner consumption (hereinafter termed a “video count ATR”) are known.
Although the video ATR is advantageous from the viewpoint of the cost because toner-density detection means is unnecessary, it has the disadvantage that errors in the amount of toner replenishment are gradually accumulated. Accordingly, some way for correcting the errors is necessary, and it is currently difficult to use the video count ATR by itself.
As described above, it is necessary to provide detection means in the developing device. At the same time, it is desirable to reduce the size of the developing device. Accordingly, automatic toner-replenishment control means using a permeability sensor as toner-density detection means is often selected because only a space for installing the permeability sensor is required, resulting in reduction of the size of the apparatus.
The permeability sensor is disposed, for example, at part of a developer conveying channel, or the like within the developing device so that a head portion of the sensor incorporating a coil, serving as a detection unit, always contacts the developer. The intensity of a magnetic field generated when a high-frequency voltage is applied to the coil within the head changes depending on the permeability around the head. Accordingly, by measuring self-inductance of the coil itself (or mutual inductance of a separate coil for measurement), the permeability of the developers around the head can be converted into an electric output value (voltage value).
The permeability sensor is usually disposed so as to face a developer conveying member for conveying a developer by rotating. Hence, the voltage output value of the permeability sensor that detects the permeability of the developer varies in accordance with the rotation of the developer conveying member. Accordingly, when representing the detection value (detection signal) of the permeability sensor that has detected the permeability of the developer, a mean value of voltage detection values of the permeability sensor at one rotation of the developer conveying member is generally used.
Conventional automatic toner-replenishment control means using such a permeability sensor as toner-density detection means have the problem that the detection signal of the permeability sensor corresponding to apparent permeability changes due to a change in the bulk density of the developer may actually be caused by a variation in environment. That is, in a low-temperature and low-humidity environment, the amount of water contained in the developer decreases, resulting in an increase in charges of the toner caused by contact between the toner and the carrier. As a result, repulsion between the toner and the carrier in the developer increases, thereby reducing the bulk density of the developer. Inversely, in a high-temperature and high-humidity environment, the amount of water contained in the developer increases, resulting in a decrease in charges of the toner caused by contact between the toner and the carrier. As a result, repulsion between the toner and the carrier in the developer decreases, thereby increasing the bulk density of the developer. That is, although the toner density in the developer receptacle is constant, the output value of the permeability sensor varies depending on the environment.
In general, as the amount of use (hereinafter represented by a “number of printed copies”) of the developing device increases, charges (triboelectrification) of the toner in the developer tend to decrease due to degradation of the developer including the toner and the carrier. The carrier is degraded due to changes in the surface property caused by mechanical stress produced, for example, by being stirred, adherence of an additive, and the like. The toner is degraded due to addition or separation of an additive, and the like. As a result, charges (triboelectrification) of the toner decrease. In this case, the bulk density of the developer also changes, and the output value of the permeability sensor changes depending on the number of printed copies although the toner density of the developer is constant.
Conventionally, correction is performed so as to stabilize the toner density of the developer by changing a control voltage input to the permeability sensor, or changing a reference output value for the permeability sensor to be compared with the current detection value of the permeability sensor in order to obtain the amount of toner replenishment, in accordance with information relating to environment and the number of printed copies. It is therefore possible to detect the toner density without causing any problem even if the bulk density of the developer changes depending on environment and the number of printed copies (for example, refer to Japanese Patent Application Laid-Open (Kokai) No. 1-291274 (1989)).
However, even if correction is performed for the operation of automatic toner-replenishment control means using a permeability sensor as toner-density detection means, in accordance with information relating to the environment and the number of printed copies in the above-described manner, the characteristics of the developer sometimes greatly change due to variations in production conditions, differences in the type of the developer, and the like, sometimes resulting in instability of the toner density of the developer. That is, when triboelectrification of the toner in the developer is high, the output value of the permeability sensor is larger than the value for the actual toner density because the amount of the carrier per unit volume decreases. Accordingly, if the density is controlled by replenishing the toner based on the output value, the toner is not sufficiently supplied as the number of printed copies increases, resulting in a small toner density in the developer. When the toner density in the developer decreases, the image density sometimes decreases. To the contrary, when triboelectrification of the toner in the developer is low, the output value of the permeability sensor is smaller than the value for the actual toner density because the amount of the carrier per unit volume increases. Accordingly, if the density is controlled by replenishing the toner based on the output value, too much amount of the toner is supplied as the number of printed copies increases, resulting in a large toner density in the developer. When the toner density in the developer thus increases, the problems of fog, and dispersion of toner particles sometimes occur.
Accordingly, there is a need for an image forming apparatus that can properly control the toner density in a developer.